I would think twice about putting syn oil in an older engine. When I switched to syn on my 1996 MH it had about 40k I had no problem. My tow car had about 80k and the pan gasket leaked within several weeks of the oil change. I had to re torque the screws twice to stop the leak. I had the same problem with my 19 year old Kohlar generator although it took longer for the leak to show up. I will go back to regular oil on the generator to see if the leak stops.
Regular and synthetic can be mixed. You just dilute some of the better properties of synthetic. My next oil change for my generator (Hopefully this weekend if weather cooperates) will be all synthetic.